The present invention relates to a color image processing apparatus such as a full-color digital copying machine or a full-color digital printer/full-color digital facsimile which can compress image data and process digital color images and color image processing method thereof.
In recent years, efforts are being made for the development of the color laser recording technology based on an electronic photograph system especially in the field of digital full-color electronic photograph system and there are strong needs for higher image quality. To satisfy the needs for higher image quality digital image processing is executed to obtain an image faithful to the original image, and in some cases excellent reproducibility is realized.
For instance, a color copying machine having one photosensitive drum produces data for 4 colors of Y (yellow), M (magenta), C (cyan), and K (Black) from document data read with a scanner, successively prepares blocks for 4 colors, and produce a full-color image by superimposing the blocks successively. In this case, color adjustment is executed for color separation signals for B (Blue), G (Green), and R (Red) produced by optically reading a document with a color CCD line sensor and the signals are then converted to the Y, M, and C signals. Then preparation of a ink block (K) and removal of the ground are executed, removal of moire and mesh dots and edge emphasis are executed, and, the charged photosensitive drum is exposed to (optically written by) a laser beam modulated according to data subjected to gradation processing after gamma correction to form an image for each color.
However, with an image forming apparatus not having a memory adequate for storing data for one page therein, scanning (for optically reading a document) must be executed 4 times, and area separation is executed each time. However, a result of area separation executed 4 times for Y, M, C, and K is not always identical due to vibration or noises, which may sometimes disadvantageously cause trouble relating to image quality such as intermittent black characters (partial lack thereof).
As the technology for solving the problem as described above, for instance, Japanese Patent Laid-Open Publication No. HEI 5-145751 discloses a xe2x80x9carea recognition signal processing circuit for a color image processing apparatusxe2x80x9d. In this circuit, by providing a page memory of one page for storing data concerning a result of area separation obtained in the first block, a difference between results of printing 4 black copies is eliminated to solve the problem as described above.
On the contrary, in the image forming apparatus having a page memory for storing data for one page therein, image data is stored simultaneously when a first block is printed, and blocks subsequent to the second block are printed by reading the data from the page memory. Further, by providing a page memory in place of a memory for storing therein a result of area separation, a result of area separation is equalized for all blocks to solve the problem described above.
The image forming apparatus having a page memory for storing data for one page has the merit that it can rotate or edit an image, and as scanning is required to be executed only once for a document, power consumption and generation of noise during operation can be suppressed and the printing speed is higher.
In the conventional type of technology as disclosed in Japanese Patent Laid-Open Publication No. HEI 5-145751, however, as a separate memory for storing a result of area separation is required the cost of the apparatus as a whole increases. In addition, since a page memory is not provided therein, the merits as described above disappear.
Further, in the image forming apparatus having a page memory, a memory capacity for storing data for one page is very large, which also results in cost increase. For instance, when the resolution is 600 dpi, the graduation is 24 bit/pixel, and the size of the paper is A3, a memory capacity of 256 mega bytes is required. One method of cutting down the required memory capacity is the application of the technology of data compression. However, if data read out from the page memory is not the same as the one used when the first block is printed, a result of area separation is not identical for all blocks, and in such a case, reverse compression for restoring the data to the original one is required. Namely, although image quality is improved by means of reverse compression, the compression ratio can not be made higher as compared to that in non-reverse compression, so that the above-described object of reduction of a required memory capacity can not be achieved.
It is an object of the present invention to provide a color image processing apparatus capable of suppressing image degradation and having a low cost and high speed page memory and color image processing method thereof.
With the present invention, a area separating unit executes processing according to the restored signals (Bxe2x80x2, Gxe2x80x2, Rxe2x80x2) for all the blocks of Y, M, C, and K, and hence a difference in a result of area separation which would otherwise easily be generated can be suppressed.
With the present invention, by executing image processing without compressing or extending a K (black) block which has a largest influence on the image quality, image degradation due to compression or extending is suppressed, and at the same time, by using restored signals after compression for YMC blocks which have a less influencing as compared to the K block copy, the compression ratio is made higher so that the required page memory can be reduced.
With the present invention, when the area separating unit generates an area separation signal, the area separating unit counts runs of Y, M, and C signals each exceeding a specified value for each pixel, and measures a run-length for each block according to the count. As an inter-character space becomes longer as compared to the pitch of the dots in the character area while the block length becomes shorter, so that a character area can easily be separated from a meshed area.
With the present invention, a compressing/extending unit subjects the BGR input signals to linear conversion so as to convert the signals to color space signals Y, I, Q each for a color difference, and then subjects the signals to sub-band conversion, so that a statistical power of an image is concentrated to the brightness signal Y and entropy can be reduced.
When the present invention, when compressed data is extended, sub-band reverse conversion is executed using each quantization coefficient to obtain Yxe2x80x2, Ixe2x80x2, and Qxe2x80x2 signals, and the compressed data is expressed by reversibly converting the Bxe2x80x2, Gxe2x80x2, and Rxe2x80x2 signals.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.